On the standard telephone keypad letters of the alphabet are associated in groups to keys, and presented in alphabetic order. The key associated to the number 2 is also associated to the letters a,b, and c, the key associated to the number 3 is also associated to the letters d,e, and f, and so on. On cellular telephones, designed to be used in any of a variety of languages, the same keys may be selectably associated to a group of letters and accented letters appropriate to the language. For example, for a phone designed to be used in French, the key associated to the number 2 maybe also be associated to the letters a,b, and c, as well as the accented letters a and a, and also the letter c{character pullout}. The standard method for accessing these letters is to tap the key a number of times equal to the position of the desired letter in some standard ordering. For example, if the ordering in the example above is a,b,c,a,a,c{character pullout}, then the user must tap the corresponding key 6 times to access the letter c{character pullout}. Thus the method is referred to as a multi-tap method. This method has the advantage of being simple to learn, of predictable behavior, and requiring software and hardware of minimal complexity to implement, and these advantages have lead to wide-spread acceptance of this method. However, the method has the disadvantage of requiring more than one keystroke on average to type most texts, and for certain symbols at least, may require a large number of taps per symbol. Most users find these large numbers of keystrokes a burden to perform, and any method which reduces the number of keystrokes is thus of great utility. The present invention teaches such a method, which preserves the advantages of the prior-art multi-tap method of being simple to learn, and requiring minimal software and hardware. It has slightly less predictable behavior than the standard multi-tap method, but this drawback is largely compensated for by the vastly reduced number of keystrokes. This method is generally applicable to any language in which strings of symbols in the language are not typically random, this includes for instance all written natural languages, as well as computer languages, and most synthetic, man-made languages, such as Esperanto and Klingon.
The essential aspect of the invention is to present the letters associated to each input means in the order in which they are most likely to be selected by the user. This order can be determined by analysis of the probability of symbols and sequences of symbols in the language. Indeed, a hierarchy of probabilistic data can be collected, permitting increasingly refined estimates of the likelihood of an ordering.
Prior-art methods to reduce the number of keystrokes using word- or block-based predictive text input, such as the methods described in U.S. Pat. No. 5,818,437 or US provisional application number 60/111,665, inevitably result in unstable displays where the letter which should be displayed at the moment a key is pressed can sometimes not be fully and correctly determined. In these cases, a letter is chosen for provisional display, and further information collected from later keystrokes is used to change the provisionally displayed letter. This effect can be disturbing to users and is a departure from the behavior of the well-known multi-tap method. In order to retain the display stability of the multi-tap method, the present invention does not allow information from subsequent keystrokes to result in changes to symbols displayed as the result of previous keystrokes.